Do referees favour big football teams?
Famous football teams benefit from favourable refereeing decisions when playing at home, it has been claimed this week. Is that true?
That was the complaint after Fulham were denied a penalty against Manchester United in a Premier League 1-0 defeat at Old Trafford, Manchester, last Monday.
The Fulham manager Martin Jol said afterwards that referees needed to be "brave" to give a visiting team a penalty at Old Trafford.
And two days later, Manchester City executive Patrick Vieira gave voice to a widely held belief when he said: "When United play at home, they may get some advantage that some other teams do not get."
The assumption among some fans has long been that big refereeing decisions tend to favour the big clubs when they play in front of their large and fervent home crowd.
"If you go to Spain it's the same, if you go to Italy, it's the same," said Vieira, who played for Arsenal for many years.
Emphasising that he hadn't seen the incident in the Fulham game and wasn't criticising Manchester Utd, he added: "It's something the teams who are used to winning get all the time, so we need to win games so we may have this kind of advantage in the future."
So is there any evidence that referees are more likely to make penalty decisions that favour big teams playing at home?
Among the current English Premier League teams which have played 50 or more home games since 2006, the statistics supplied by the sports data specialists Opta show that, on average, Manchester United have conceded one penalty every 12 home games.
This is more than Fulham, who have conceded roughly one penalty every 14 games at home.
So it is actually more common for a visiting team to get a penalty at Old Trafford than Craven Cottage.
Fulham are in fact among seven current Premier League clubs who have since 2006 conceded fewer penalties at home than Man Utd.
It's true that the team in the Premier League that has conceded the most penalties on average (one penalty every six matches) is one of the smaller clubs, Blackburn Rovers.
And also the team that has conceded the fewest (one penalty only every 18 games) is Chelsea, one of the top dogs. They have also won a penalty on average once every five games.
But there could be a far simpler explanation than referee bias, says a sports correspondent with a keen eye on the statistics, Bill Edgar of the Times newspaper.
"Given that Chelsea have been among the most successful clubs over that period, you would have expected them to have given away the fewest penalties simply because the opposition are generally weaker and therefore will spend less time in Chelsea's penalty area, so there will be less opportunity for them to win penalties," he says.
If the big clubs aren't necessarily conceding fewer penalties because of the referee, do they maybe have penalties awarded in their favour?
Manchester United have been awarded a penalty every 4.4 games, on average. This makes them the second-placed team in terms of the number of penalties won.
The first? Patrick Vieira's Manchester City. They were awarded a penalty every 3.93 games they played on average.
The two Manchester teams are the top two this season, battling it out for the Premier League trophy.
The team that ranks lowest in terms of the number of penalties won is Wolverhampton Wanderers who were awarded a penalty only every 13 matches on average.
But Vieira also complains that refs are biased towards the big clubs in Spain and Italy. And Real Madrid's manager Jose Mourinho is frequently up in arms over what he sees as bias in favour of Barcelona.
For the BBC, the sports statisticians at Opta have looked at every La Liga match involving Barcelona and Real Madrid since 2006.
The stats show that Real Madrid conceded one penalty roughly every 11 games in front of their home crowd, while Barcelona conceded one penalty in every 10 games.
So it's Jose Mourinho's own team, Real Madrid, who have a better penalty average.
And Real Madrid didn't just concede fewer penalties at home - they won more too - roughly one every four home games, compared with Barcelona's one in five.
But statistics alone do not tell us the truth about whether big teams really are favoured in the penalty area, says Edgar, who has an interesting solution.
"The only way you could really decide once and for all is to take disputed penalty instances at the home of each different Premier League team," he says.
"Then collect a panel of football experts and let them watch them. You would have the players' identities removed and the teams' identities removed and compare [the panel's] consensus with the decision the referees made.
"And if there was a considerable difference, then that would point to a bias among referees."
This kind of anonymised approach is possible in some fields. Orchestras famously introduced blind auditions where musicians played behind screens. When they did, the number of women hired rose sharply.
But it would be somewhat trickier to conduct blind experiments involving referees.
Mind you, some football fans will tell you the ref's blind anyway.